1. Field of the Invention
The present invention relates to a method for manufacturing a spark plug and an apparatus for carrying out the same.
2. Description of the Related Art
Conventionally, in manufacture of a so-called parallel-electrode-type spark plug, a spark gap is formed and adjusted in the following manner: after a ground electrode is subjected to preliminary pressing, the ground electrode is repeatedly subjected to pressing while the gap is being monitored by use of a CCD camera or a like device, until the gap reaches a target value.
3. Problems to be Solved by the Invention
When a gap is to be monitored by use of a CCD camera or a like device for the purpose of adjusting the gap, the installation direction of a spark plug (specifically, the direction of the axis of a center electrode) is determined so as to comply with a coordinate system of an image obtained through photographing. That is, when a measurement technique to be used is such that the direction of any one coordinate axis (e.g., the Y direction) of the coordinate system coincides with the direction of the center electrode, a gap is obtained by measuring the distance along the coincident direction between edges of the center electrode and a ground electrode.
However, as shown in FIG. 20, when a workpiece W is photographed while the axis of the center electrode is inclined with respect to a reference direction of the gap measurement (the Y direction in the figure) (specifically, while the axis is inclined laterally with respect to the direction of photographing by photographing means), the direction along which a gap must be measured is inclined with respect to the reference direction. Accordingly, the inclination may result in a dimensional error between an actual value gr and measured value gxe2x80x3 obtained from the image. Also, as shown in FIG. 8, when the workpiece is photographed while the axis is inclined along the direction of photographing by the photographing means, a dimensional error may similarly arise.
An object of the present invention is to provide a method for manufacturing a spark plug that can determine an accurate gap regardless of inclination of a workpiece (a spark plug) with respect to the measuring means and can manufacture the spark plug at high accuracy by use of the calculated gap, as well as to provide an apparatus for carrying out the same.
The above object of the present invention has been achieved by providing a method (and apparatus) for manufacturing a spark plug comprising a center electrode disposed within an insulator, a metallic shell disposed outside the insulator, and a ground electrode, one end of the ground electrode being joined to an end face of the metallic shell, an opposite end portion of the ground electrode being bent such that a side surface of the opposite end portion faces an end face of the center electrode so as to form a spark gap between the side surface and the end face, said method (apparatus) comprising:
a photographing step (photographing means) for photographing the spark gap;
a gap calculation step (gap calculation means) for determining a gap which comprises defining a reference point on the basis of image information obtained from said photographing step, defining a plurality of measurement lines passing through the reference point, measuring along each of the measurement lines a distance between a ground electrode spark gap definition portion of the ground electrode facing the spark gap and a center electrode spark gap definition portion of the center electrode facing the spark gap, and determining a gap on the basis of the measured distance; and
an after-treatment step (after-treatment means) for performing a predetermined after-treatment on the basis of the calculated gap.
Since the gap is determined in the above-described manner, the gap can be measured accurately, even when a spark plug is photographed in such a manner as to be inclined on the captured image as shown in FIG. 20; i.e., even when the axis of the center electrode of the spark plug is inclined laterally on the plane of the image. That is, the inclination of a spark plug on the plane of image does not result in a dimensional error.
The present invention further provides a method (apparatus) for manufacturing a spark plug comprising a center electrode disposed within an insulator, a metallic shell disposed outside the insulator, and a ground electrode, one end of the ground electrode being joined to an end face of the metallic shell, an opposite end portion of the ground electrode being bent such that a side surface of the opposite end portion faces an end face of the center electrode so as to form a spark gap between the side surface and the end face, the method (apparatus) comprising:
a photographing step (photographing means) for photographing the spark gap;
a gap calculation step (gap calculation step) for determining a gap which comprises determining, on the basis of image information obtained from the photographing step, a single reference point on the outline of either a ground electrode spark gap definition portion of the ground electrode facing the spark gap or a center electrode spark gap definition portion of the center electrode facing the spark gap, finding a measurement point on the outline of the other spark gap definition portion such that the distance between the reference point and the measurement point is the shortest, and determining the gap on the basis of the shortest distance; and
an after-treatment step (after-treatment means) for performing a predetermined after-treatment on the basis of the calculated gap.
Since the gap is determined in the above-described manner, the shortest distance across the gap can be obtained at high accuracy. That is, the inclination of a spark plug on the plane of image does not result in a dimensional error, thereby contributing to highly accurate gap adjustment.
Alternatively, the method for manufacturing a spark plug may comprise obtaining an apparent size of a gap (hereinafter also called an xe2x80x9capparent gap sizexe2x80x9d) as observed on an image obtained through photographing; and correcting the apparent gap size on the basis of an apparent dimension of a predetermined measurement reference portion of the spark plug (hereinafter also called a xe2x80x9cmeasurement reference portion apparent-dimensionxe2x80x9d) as observed on the captured image and a known standard dimension of the measurement reference portion (hereinafter also called a xe2x80x9cmeasurement reference portion standard-dimensionxe2x80x9d), to thereby calculate the gap. Specifically, for example, the method corrects a dimensional error in the apparent gap size associated with the spark plug being photographed while being inclined along the direction of photographing by photographing means, on the basis of the measured reference portion apparent-dimension and the measured reference portion standard-dimension.
According to the method described above, even when a spark plug is photographed while being inclined along the direction of photographing by photographing means, a value that is very close to an actual dimension can be obtained by correction, thereby enabling highly accurate establishment of a gap. Combined use of the above method and the previously described method, which calculates a gap on the basis of a measurement point and a reference point on outlines, can cope with the inclination of a spark plug along the direction of photographing and in a lateral direction with respect to the direction of photographing.